Angled-wedge chrome-face wall for intensity balance of alternating phase shift mask

ABSTRACT

A method for forming a semiconductor device is presented. The method includes providing a substrate having a photoresist thereon and transmitting a light source through a mask having a pattern onto the photoresist. The mask comprises a mask substrate having first, second and third regions, the third region is disposed between the first and second regions. The mask also includes a light reducing layer over the mask substrate having a first opening over the first region and a second opening over the second region. The first and second openings have layer sidewalls. The sidewalls of the light reducing layer are slanted at an angle less than 90 degrees with the plane of a top surface of the mask substrate. The method also includes developing the photoresist to transfer the pattern of the mask to the photoresist.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application which claims benefit ofcopending U.S. patent application Ser. No. 11/297,532, filed on Dec. 7,2005. All disclosures are incorporated herewith by reference.

BACKGROUND OF INVENTION

1) Field of the Invention

This invention relates generally to structures and method of makingcircuit fabrication masks and more particularly some embodiments relateto the structures and methods of making of phase shifting circuitfabrication masks.

2) Description of the Prior Art

Improvements in photolithography have increased the density and enhancedthe performance of semiconductor devices by shrinking integratedcircuits (ICs). As described by the Rayleigh criterion, the minimumcritical dimension (CD) which can be resolved by a wafer stepper isdirectly proportional to the wavelength of the illumination source andinversely proportional to the numerical aperture (NA) of the projectionlens. However, diffraction tends to degrade the aerial image when the CDbecomes smaller than the actinic wavelength. The actinic wavelength isthe wavelength of light at which a mask is used in a wafer stepper toselectively expose photoresist coated on a substrate, such as a Siliconwafer. As needed, a resolution enhancement technique (RET), such as aphase-shifting mask (PSM), may be used to achieve a wider processlatitude. Unlike a binary mask that only uses Chrome to control theamplitude of light transmitted through a quartz substrate, a PSM furthermodulates the phase of light to take advantage of destructiveinterference to compensate for the effects of diffraction.

An alternating PSM (AltPSM) is a type of PSM that is particularlyhelpful in improving contrast when patterning very small CDs, such asthe gate length of a transistor in a device. AltPSM introduces a phaseshift of 180 degrees between the light transmitted through adjacentclear openings so destructive interference can force the amplitudebetween the two images to zero. A phase shift of 180 degrees isimplemented by creating a difference in the optical path lengths throughadjacent openings in an opaque layer, such as Chrome. A subtractiveprocess may be used to etch a trench into the quartz substrate inalternate openings. However, incident light may scatter off thesidewalls and bottom corners of the etched trench and cause an imbalancein the aerial image that varies as a function of focus. Such a waveguideeffect may be manifested as a CD error and a placement error.

The intensity and phase in the aerial image of an AltPSM may be balancedin various ways. A selective biasing approach enlarges the CD of theetched opening relative to the unetched opening to balance the aerialimage. An etchback approach undercuts the edges of the chrome in bothopenings to balance the aerial image. A dual-trench approach etches adeep trench in the phase-shifted opening and a shallow trench in thenon-phase-shifted opening to balance the aerial image.

The basic concept of increasing the resolution of a lithographic imageis to modify the optical phase of the mask transmission. In analternating PSM, alternating areas of chrome and 180 degree-shiftedquartz are employed to form features on the wafer. Contrast is increasedbecause the light diffracted into the nominally dark area will interferedestructively with the light diffracted from the clear area. The altPSMis the “strongest” PSM technology and can improve the resolution of agiven wafer exposure system by approximately 40%.

The problem of aerial image intensity imbalance through focus withaltPSM has been well-documented. One known solution for the intensityimbalance issue is to provide undercuts beneath the chrome and to bias(i.e., to thin) the trench chrome opening. However, undercuts andbiasing limit the minimum chrome size and hence contributes to thechrome peeling issue.

FIG. 9 shows some examples of the methods used to correct the imageimbalance. The graph on the left shows the intensity imbalance for thePSM (shows below on the left.). The graph on the right shows improvedimage balance. Below are 3 examples of methods to improve; 1) undercut,2) bias and 3) undercut an bias.

The apparently more relevant technical developments in the patentliterature can be gleaned by considering the following patents.

U.S. Pat. No. 6,531,250: Kim and US20010009745A1: Kim—Half tone phaseshift mask having a stepped aperture—Half-tone phase shift mask used informing predetermined pattern of semiconductor integrated circuits,includes light-transmitting phase shift pattern defining steppedaperture.

U.S. Pat. No. 5,514,500: Ham—Half-tone type phase shift mask and methodfor fabricating the same—Mfr. of half-tone type phase shift masks—byforming phase shift layer on transparent substrate, forming light screenon phase shift layer, selectively etching light screen and phase shiftlayer to form pattern. The chrome pattern has a step.

U.S. Pat. No. 5,281,500:—Cathey, David A.;—Method of preventing nullformation in phase shifted photomasks—Preventing zero formation inphase-displacement photoresist masks—by proving a transparent substratewith dark features in relief, and delimiting at least one end ofupwardly-projecting clear phase displacement features, etc.

U.S. Pat. No. 5,487,962:—Rolfson, J. Brett;—Method of chromeless phaseshift mask fabrication suitable for auto-cad layout—: Chrome-lessphase-shift masks suitable for auto CAD layout—comprising raisedshifters formed with vertical edge and tapered edge.

US20040073884A1:—Kroyan-Phase shifting mask topography effect correctionbased on near-field image properties—Image intensity imbalancecorrection method for phase shifting mask in e.g. deep UV lithography,involves computing near-field image for pair of shifters, based on whichbias is determined for phase shifters.—The patent shows undercuts for analt-psm.

U.S. Pat. No. 6,458,495:—Tsai, Wilman;—Transmission and phase balancefor phase-shifting mask—Phase-shifting mask with balanced transmissionand phase, has trenches with vertical sidewall profiles which areretrograde

U.S. Pat. No. 6,627,359:—Kokubo, Haruo—Phase-shift photomaskmanufacturing method and phase-shift photomask—Phase-shift photomaskmanufacture for forming resist pattern on wafer, involves wet-etchinglight transmission section on substrate to set depth of recesses formedon section to satisfy specific relationship.

U.S. Pat. No. 6,410,191:—Nistler, John L.—Phase-shift photomask forpatterning high density features.—Phase-shift photomask for patterninghigh density features and manufacture. The phase shifting regionincludes sloped sidewalls having a slope of less than about 85°.

SUMMARY OF THE INVENTION

The embodiments of the present invention provides a structure and amethod of manufacturing a mask which is characterized as follows.

An example embodiment is a mask comprising:

-   a substrate having a first region, a second region and a third    region; the third region positioned between the first region and the    second regions; the substrate has a first surface;-   an light reducing layer over the substrate having a first opening    over the first region and a second opening over the second region;    the first opening and the second opening having light reducing layer    sidewalls;-   the sidewalls of the light reducing layer are slanted at an angle    less than 90 degrees with the plane of the top surface of the    substrate.

Another example embodiment is a method for forming a mask comprising:

-   -   a) forming an light reducing layer over a substrate;        -   (1) a substrate having a first region, a second region and a            third region; the third region position between the first            region and the second regions; the substrate has a first            surface;        -   (2) the light reducing layer has a first opening over the            first region and a second opening over the second region;            the first opening and the second opening defined by the            light reducing layer sidewalls;            -   (a) the sidewalls of the light reducing layer have a                sidewall angle with the plane of the top surface of the                substrate; the sidewall angle is about 90 degrees;    -   b) forming a first trench in the first region;    -   c) etching the light reducing layer sidewalls to make the        sidewall angle less than 90 degrees.

Additional example embodiments are further described in the claims asfiled and amended during prosecution and in the specification below.

The above and below advantages and features are of representativeembodiments only, and are not exhaustive and/or exclusive. They arepresented only to assist in understanding the invention. It should beunderstood that they are not representative of all the inventionsdefined by the claims, to be considered limitations on the invention asdefined by the claims, or limitations on equivalents to the claims. Forinstance, some of these advantages may be mutually contradictory, inthat they cannot be simultaneously present in a single embodiment.Similarly, some advantages are applicable to one aspect of theinvention, and inapplicable to others. Furthermore, certain aspects ofthe claimed invention have not been discussed herein. However, noinference should be drawn regarding those discussed herein relative tothose not discussed herein other than for purposes of space and reducingrepetition. Thus, this summary of features and advantages should not beconsidered dispositive in determining equivalence. Additional featuresand advantages of the invention will become apparent in the followingdescription, from the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of a mask according to the present inventionand further details of a process of fabricating such a mask inaccordance with the present invention will be more clearly understoodfrom the following description taken in conjunction with theaccompanying drawings in which like reference numerals designate similaror corresponding elements, regions and portions and in which:

FIG. 1 shows a cross sectional view of an example embodiment of an maskwith opaque layer with sloped sidewall openings according to an exampleembodiment of the present invention.

FIG. 2 shows the plane diffraction at a corner of the opaque layer inthe openings.

FIGS. 3A to 3G show an example method for forming a mask having slantedopaque layer sidewall according to an example embodiment of the presentinvention.

FIGS. 4A and 4B shows graphs of diffraction field vs angle ofdiffraction for a range of sidewall angles according to an exampleembodiment of the present invention.

FIGS. 5A and 5B shows graphs of diffraction field vs angle ofdiffraction for a range of sidewall angles according to an exampleembodiment of the present invention.

FIGS. 6A, 6B, 6C and 6D show graphs of (a) image intensity vs locationfor the best focus and (b) (a) image intensity vs location for the bestfocus and Focus at −0.2 microns for:

-   -   (1) single trench PSM with straight trench sidewalls.        (anisotropic trench etch)    -   (2) single trench PSM with curved trench sidewalls. (isotropic        trench etch)    -   (3) single undercut trench PSM (anisotropic+isotropic trench        etch)    -   (4) the embodiment's angled chrome sidewall mask.

FIG. 7A shows SSCAAM: Sloped Sidewall Chrome Alternating Aperture Maskusing the embodiment sloped sidewall opaque layer according to anexample embodiment of the present invention.

FIG. 7B shows a Dual-trench Sloped Sidewall Chrome using the embodimentsloped sidewall opaque layer according to an example embodiment of thepresent invention.

FIG. 7C shows Single-trench with undercut using the embodiment slopedsidewall opaque layer according to an example embodiment of the presentinvention.

FIG. 7D shows a double trench Sloped Sidewall Chrome with undercutaccording to an example embodiment of the present invention.

FIG. 8A shows an example of a curved sloped sidewall light reducinglayer according to an example embodiment of the present invention.

FIG. 8B shows another example of a curved sloped sidewall light reducinglayer according to an example embodiment of the present invention.

FIG. 9 shows some examples of the methods used to correct the imageimbalance according to the prior art.

DETAILED DESCRIPTION OF THE NON-LIMITING EXAMPLE EMBODIMENTS

The example embodiments of the present invention will be described indetail with reference to the accompanying drawings. The exampleembodiments provide structures and methods of forming a mask used insemiconductor device manufacturing.

B. Introduction

FIG. 9 shows some prior art examples of the methods used to correct theimage imbalance in phase shift masks (PSM). The graph on the left showsthe intensity imbalance for the PSM (shows below on the left.). Thegraph on the right shows improved image balance. Below are 3 examples ofmethods to improve; 1) undercut, 2) bias and 3) undercut an bias.

Some of the example embodiments can help correct the image imbalance inphase shift masks by using an opaque or half tone layer that has slopedface sidewalls (e.g., angled-wedge face). FIG. 1 shows an example of aphase shift masks by using an opaque or half tone layer having slopedface sidewalls.

Terms:

-   light reducing layer—a layer that reduces the amount of light    passing through. A light reducing layer can be a half tone (e.g.,    translucent) or an opaque layer (e.g., chrome).

C. Mask with Opaque Layer with Sloped Sidewall Openings

An example embodiment of a mask used in semiconductor manufacturing isshown in FIG. 1. FIG. 1 shows a cross sectional view of an Mask withlight reducing layer 30 (e.g., opaque or half tone layer 30) with slopedsidewall openings.

In the description below the light reducing layer 30 is referred to as aopaque layer 30. However, layer 30 can be a light reducing layer 30 suchas a opaque or half tone layer 30.

The mask 11 comprises: a substrate 10 having a first region 14, a secondregion 18 and a third region 22. The third region 22 is positionedbetween the first region 14 and the second regions 18.

The substrate 10 has a first surface 10A that an opaque layer (e.g.,light shielding layer) is formed over.

An opaque layer 30 is formed over the substrate 10. The opaque layer hasa first opening 36 over the first region 14 and a second opening 34 overthe second region 18. The first opening 36 and the second opening 34 aredefined by opaque layer sidewalls 36A and 34A respectively.

The sidewalls 34A 36A of the opaque layer are slanted at an sidewallangle 31 less than 90 degrees with the plane of the top surface of thesubstrate.

The sidewalls of the opaque layer are slanted at an angle 31 with theplane of the top surface of the substrate (sidewall angle) between 5 and89 degrees and more preferably between 30 and 60 degrees.

The openings are smallest at the first surface and the opening get widerat the top of the opaque layer.

In an option, light 26 transmitted through the first region 14 and thesecond region 18 are about 180 degrees out of phase.

Light transmitted through the first region has a first phase shift andlight transmitted through the second region has a second phase shift.The first phase shift is preferably about 180 degrees from the secondphase shift. The first region can be referred to as an unshifted regionand the second region can be referred to as a phase shifted region.

In an option (single trench PSM) shown in FIG. 1, the first region 14preferably comprises a first trench 40.

In another option (dual trench PSM) shown in FIG. 3G, the first region14 comprises a first trench 40 and the second region 18 comprises asecond trench 44. The light passing through the first and secondtrenches is preferably about 180 degrees out of phase.

D. Some Explanation for Opaque Layers with Sloped Sidewall Openings

FIG. 1 shows light 26A diffracted off the corner to the opaque layer atdifferent angles.

From FIG. 1, we see that the most possible (We choose our study at 180degrees diffraction angle (θ) because the optical path for any twonearest opaque layer sidewall is the shortest, and thus would determineits phase and whether the interference will be constructive ordestructive; not because it is the largest diffraction angle)diffraction angle for optical interference between two nearest opaquelayer sidewalls is 180 degrees (θ). At 180 degrees diffraction angle,the optical path for any two nearest feature edges is the shortest. Toachieve a balanced aerial image intensity, I₀=I₁₈₀. (balanced aerialimage intensity is defined when light 26 transmitted through the firstregion 14 and the second region 18 are about 180 degrees (φ)̂̂ out ofphase and of the same intensity) ̂where φ refers to the phase differenceof the light reference to the phase angle passing through the unetchedquartz surface.

-   -   **Where θ is the diffraction angle. 0 deg refer to the angle        where the quartz interfaces with the chrome boundary towards the        primary chrome feature. 90 deg refer to the direction        perpendicular to the quartz surface into the quartz, away from        the chrome. 180 deg refer to angle of the shortest displacement        (distance) of the adjacent chrome feature. Hence the maximum is        360 deg.

In typical use, light 26 is passed through the mask 11 by passingthrough a second (e.g., top) side of the substrate 10 and out the firstsurface 10A (bottom) side of the substrate 10 through the openings 34and 36 in the opaque layer 30. The light shines on a photoresist toexpose a pattern on the photoresist.

E. Plane Diffraction at a Wedge (Chrome Sidewall Openings)

FIG. 2 shows the plane diffraction at a corner of the opaque layer inthe openings.

FIG. 2 shows that we have used Geometrical Theory of Diffraction tocharacterize and analysis the light diffraction phenomenon for differentdiffraction angle 0 for different sidewall angle β.

In FIG. 2, the following are explained:

-   region 1: region space 1: only diffracted field exist in this region    (where β is the side wall angle 31 and Chrome exist in the boundary    from 0 to β, so from β to “shadow boundary” is air. Region space 1    consist of Chrome and air) shadow boundary is define by the light    path passing from the quartz to the air and shadow edge of the    chrome feature blocking the light.-   region 2: region space 2 is separated from region 1 by the shadow    boundary: geometrical and diffracted field exist.-   Region 2 starts from the shadow boundary up to the reflection    boundary where air and quartz exist for 2^(nd) region of FIG. 1 (18)    and air only for 1^(st) region of FIG. 1 (14))-   reflection boundary: The reflection boundary is defined by the path    of the reflected light on the interface of the quartz, chrome and    air traveling from the quartz towards the air and reflect back into    the quartz.-   region 3: region space 3 from wedge face separated from region 2 by    reflection boundary: incident and reflected wave exist.-   Region 3 refers to the region between region 1 and 2. Starts from    the reflection boundary to the Chrome and quartz interface.

We are working in region 2 for diffraction angle 180 degrees (θ).

When a plane wave is normally incident upon a corner of the opaque layerin the openings (i.e. incident wavefront is parallel to edge), as shownin FIG. 2, and has a field component V_(z) ^(i) in the z-direction suchthat

V _(z) ^(i)=exp{jk(x cos θ₀ +y sin θ₀)}  (1)

where θ₀ is the angle of incidence to the half-plane. The exact solutionfor the z-component at any field point (ρ,θ) (where ρ is the opticaldistance away from point of diffraction) can be written in compact formas followed:

V _(z)(ρ,η)=u ^(i)(ρ,θ)∓u ^(r)(ρ,θ)   (2)

The upper sign is for electric polarization (TE case) when V_(z)^(i)=E_(z) ^(i) and hence Eq. (2) expresses the total electric field.For the lower sign we have magnetic polarization (TM case) where V_(z)^(i)=H_(z) ^(i) and Eq. (2) now yields the total magnetic field. Thesuperscript i or r indicates that the particular field component isassociated with the incident or reflected geometrical optics field.These components in Eq. (2) are given by

u ^(i,r)(ρ,θ)=U(ε^(i,r))u _(o) ^(i,r)(ρ,θ)+u _(d) ^(i,r)(ρ,θ)   (3)

where U is the unit step function (1 for ε>0, 0 otherwise), u_(o) ^(i,r)is the geometrical optics field For the geometrical optics field,

$\begin{matrix}{{u_{o}^{i,r}( {\rho,\theta} )} = {\exp \{ {j\; k\; {{\rho cos}( {{\theta \mp \theta_{0}} + {2n\; \pi \; N}} )}} \}}} & (4) \\{N = \frac{{2\pi} - \beta}{\pi}} & (5)\end{matrix}$

where n is an integer, which satisfies |θ∓θ₀+2Nnπ|<π and β is thesidewall angle 31.and u_(d) ^(i,r) is the edge diffracted field

$\begin{matrix}{{{u_{d}^{i,r}( {\rho,\theta} )} = {{- ɛ^{i,r}}K\{ {{a^{i,r}}\sqrt{( {k\; \rho} )}} \} {\exp ( {{- j}\; k\; \rho} )}}},{a^{i,r} = {\sqrt{2}\cos \frac{1}{2}( {\theta \mp \theta_{0}} )}},{ɛ^{i,r} = {{sgn}( a^{i,r} )}},{{{K\_}(s)} = {\sqrt{\frac{j}{\pi}}{\exp ( {j\; s^{2}} )}{\int_{s}^{\infty}{{\exp ( {{- j}\; t^{2}} )}{t}}}}}} & (6)\end{matrix}$

Away from the optical boundaries and the edge, i.e. at far field (=10⁶λ)or Fraunhofer region, the edge diffraction field can be given in generalas: For TE (electric polarization),

$\begin{matrix}{{{{ E_{z}^{d} \sim{D^{e}( {\theta,\theta_{0}} )}}\frac{\exp ( {{- j}\; k\; \rho} )}{\sqrt{( {8\; j\; k\; \pi} )}}},{{ H_{\theta}^{d} \sim{- \sqrt{\frac{ɛ}{\mu}}}}E_{z}^{d}}}{ H_{p}^{d} \sim 0}} & (7)\end{matrix}$

For TM (magnetic polarization),

$\begin{matrix}{{{ E_{\theta}^{d} \sim\sqrt{\frac{\mu}{ɛ}}}H_{z}^{d}}{ E_{p}^{d} \sim 0}} & (8)\end{matrix}$

where D^(e)(θ,θ₀) and D^(m)(θ,θ₀) are known as the edge diffractioncoefficients, μ and ε are the permeability and permittivity of medium.

In this study, diffraction coefficients derived from the canonicalproblem are multiplied with the incident ray at the point of diffractionto produce a diffracted field on the diffracted rays.

U_(d) ^(e,m)≈D^(e,m).V_(z) ^(i)   (9)

Away from the optical boundaries and the edge, i.e. at far field (=10⁶λ)or Fraunhofer region, the edge diffraction coefficients for wedgediffraction in general can be approximated by:

$\begin{matrix}{D^{e,m} = {\frac{2}{N}\sin \frac{\pi}{N}\begin{Bmatrix}{( {{\cos \frac{\pi}{N}} - {\cos \frac{\theta - \theta_{0}}{N}}} )^{- 1} \mp} \\( {{\cos \frac{\pi}{N}} - {\cos \frac{\theta + \theta_{0}}{N}}} )^{- 1}\end{Bmatrix}}} & (10)\end{matrix}$

The formulation above allows us to determine the diffracted field forboth TE and TM polarization for 180 degree diffraction angle (θ₀=180deg) using conventional illumination (θ=90 deg) for different sidewallangle β (use equation 5 to find out N).

Angle “β” is the sidewall angle 31 in FIG. 1.

F. Image Intensity Graph Results—FIGS. 6A-6D

FIGS. 6A, 6B, 6C and 6D show graphs of (a) image intensity vs locationfor the best focus and (b) (a) image intensity vs location for the bestfocus and Focus at −0.2 microns for:

-   (1) FIG. 6A—single trench PSM with straight trench sidewalls.    (anisotropic trench etch)-   (2) FIG. 6B—single trench PSM with curved trench sidewalls.    (isotropic trench etch)-   (3) FIG. 6C—single undercut trench PSM (anisotropic+isotropic trench    etch)-   (4) FIG. 6D—the embodiment's angled chrome sidewall mask.

FIGS. 6A-6D show that (4) the embodiment's angled chrome sidewall maskproduces the most balanced image intensity. The data are postulated andsimulated using SOLID-C (a commercial lithography software)

The data for FIGS. 6A and 6b was obtained by simulation

II. Example Method for Forming Sloped Opaque Sidewall

An example embodiment is a method for making a mask that has slope lightreducing (e.g., half tone or opaque) sidewalls.

In the description below the light reducing layer 30 is referred to as aopaque layer 30. However, layer 30 can be a light reducing layer 30 suchas a opaque or half tone layer 30.

FIGS. 3A to 3G show an example method for forming a mask. A feature isthe slanted opaque layer sidewall. The element numbers generallycorrespond with the analogous elements in FIG. 1.

A. Form an Light Reducing Layer (e.g., Half Tone or Opaque Layer) 30Over a Substrate 10

FIG. 3A shows a cross sectional view of a mask substrate 10. Thesubstrate is preferably comprised of a light transmissive substrate,preferably transparent. The substrate can be comprised of quartz orfused silica.

We form an opaque layer 30 over a substrate 10. The opaque layer is apreferably a light blocking layer is preferably substantially opaque.The opaque layer is preferably comprised of chrome or photoresist and ismost preferably comprised of chrome with a thickness between 500 Å and1000 Å. In an option, an light reducing layer is an opaque layerpreferably blocks essentially all light from passing through.

The mask substrate 10 preferably comprises a first region 14, a secondregion 18 and a third region 22. At least a portion of the third region22 is positioned between the first region 14 and the second regions 18.The substrate 10 has a first surface that the opaque layer is formedover or on.

Referring to FIGS. 3A and 3B, we pattern the opaque layer to form afirst opening 34 over the first region 14 and a second opening 18 overthe second region 18. The first opening and the second opening definedby opaque layer sidewalls. The sidewalls of the opaque layer have aninitial sidewall angle with the plane of the top surface of thesubstrate. The sidewall angle is about 90 degrees.

In an example shown in FIG. 3A, a masking layer 32 (e.g., resist) isformed over the opaque layer. Referring to FIG. 3B, the masking layer ispatterned to form first and second masking openings. The opaque layer ispreferably anisotropically etched using the masking layer 32 as a maskto form the first opening 36 over the first region 14 and a secondopening 34 over the second region 18.

The patterns in the masking layer are typically formed utilizing somemask writing tool. An example writing tool comprises the ETEC MEBES4500, available from ETEC Systems, Inc, of Hillsboro, Oreg.

The chrome pattern 30 can be used to define a desired circuit pattern ofa semiconductor device.

FIG. 3C shows the step of removing the resist layer 32.

B. Form a First Trench 40 in the First Region

Referring 3D, we form a first trench resist layer 38 over at least thesecond region 18. The first trench resist layer 38 has an opening asleast over the first region 14.

Referring to FIG. 3E, we form a first trench 40 in the first regionpreferably using the first trench resist layer 38 as an etch mask. Thetrench can be etching using a anisotropic etch such as dry-etch processusing CHF₃+H₂ plasma under certain process conditions.

C. Etch the Opaque Layer Sidewalls to Make the Sidewall Angle Less Than90 Degrees

Next, we reform the opaque layer sidewalls to make the sidewall angleless than 90 degrees.

Referring to an example in FIG. 3F, we preferably etch the opaque layersidewalls to make the sidewall angle 31 less than 90 degrees.

The sidewall angle 31 is between 30 and 60 degrees and is morepreferably between 30 and 60 degrees.

The sidewalls are preferably etched with SF₆ dry etch.

The SF₆ etch etches the sidewall to have a straight slope by usingproper balancing of RF power, etchant gases and temperature.

The (e.g., SF₆) etch preferably does not significantly etch the trench40 any deeper.

The tip of the light reducing layer 30 where the slope edge meets thebottom, can be a shape tip or a rounded tip. Our simulation results showgood image intensity balance performance for sharp tip. Ideally, theshape of the tip of the chrome wedge must be sharp. However in real maskfabrication, there will be small rounding at the tip, which is stillacceptable.

D. Isotropic Quartz Wet Etching

Referring to FIG. 3G, we preferably etch the substrate in the first andsecond regions. The etch is preferably an isotropic quartz wet etch suchas a wet etch. The etch is to provide a smoother topography of thetrenches 40 44.

Note that the isotropic etch forms a second trench 44 in the secondregion 18 and makes the first trench 40 deeper.

The 2^(nd) trench 44 is optional.

Preferably at this point, light transmitted through the first region 14and the second region 18 are about 180 degrees out of phase.

FIG. 3G shows sidewalls 36A and 34B of opquage layer 30.

Isotropic wet etch will give a smooth-surfaced curved shape because theetch rate is uniform—as each point of the surface is removed at the samerate. For feature with corner, a curved surface will result from thecorner feature. For anisotropic etch, it is a directional controlled bythe RF power, etchant gases, timing and temperature. Hence a straightsidewall can be achieved.

III. Supporting Data—FIGS. 4A 4B 5A and 5B

The following variables are used in FIGS. 4A, 4B, 5A and 5B. β has beendefined as the chrome sidewall angle 31.

has been defined in equation (9).

We see that the most possible diffraction angle for optical interferencebetween two nearest feature edges is 180°. At 180° diffraction angle,the optical path for any two nearest feature edges is the shortest.

FIG. 4A shows normalized diffracted field |U_(d) ^(e)| forE-polarization for different N. We determine that D^(e) for N=1.5 (90°wedge, i.e. square chrome block) is the highest among other |U_(d) ^(e)|from 120° diffraction angle onwards. This shows that 90° wedge (e.g.,chrome sidewall angle 31 is 90 degrees) is more prone to opticalinterference than angled wedge. We can conclude from FIG. 4A thefollowing to minimize or eliminate the undercut that can cause chromepeeling, we can adopt suitable amount of sidewall angle β to achievebalanced aerial image. This shows that a 90° wedge is more prone tooptical interference than an angled wedge.

FIG. 4B shows the closed-up view of normalized diffracted field |U_(d)^(e)| for E-polarization for different N at 180°. We can conclude fromFIG. 4B the following: adopting a suitable amount of sidewall angle 31,we can reduce the diffracted field at 180 deg diffraction angle to closeto zero in an effort to reduce the interference between the 2 corners,regardless of the interference will be constructive or destructive.

FIG. 5A shows normalized diffracted field |U_(d) ^(m)| forH-polarization for different N. We see that |U_(d) ^(m)| for angledwedges have zero or small optical interference than square wedge fordiffraction angle ˜180° to 200°. We see that the D^(m) value for anangled wedge have zero or small optical interference compared to asquare wedge at diffraction angle of 180° to 200°. At a diffractionangle of 180°, the optical path between nearest feature edges is theshortest and constructive or destructive interference cannot be isolatedor distinguished. Nevertheless, we can reduce the diffracted field tominimize or eliminate the amount of undercut.

FIG. 5B shows the closed-up view of normalized diffracted field |U_(d)^(m)| for H-polarization for different N at 180°. We can reduce thediffracted field to minimize the amount of undercut. Hence, the angledwedge can be optimized (for an angle β) for different pitch ratio forsame amount of interference (intensity balance) at 0-degree and180-degree etched quartz. The results obtained suggest that a noveldesign for a phase shift mask with an arbitrary angled wedge on thechrome face is feasible.

A. Optional Data Biasing

The embodiment's sloped opaque sidewall is preferably implemented in analternating phase shift mask. This sloped sidewall can be combined withshifter width biasing. That is the width of the first and secondopenings can be Biased (e.g., made smaller or larger) from the initialdesign sizes to improve the mask performance.

Shifter width biasing refers to enlarge of the quartz opening to allowmore light to pass through. E.g., the term 90 nm bias on mask refers tosizing up the quartz opening by 45 nm larger per edge.

For example, the PSM image imbalance correction can be implementedthrough the application of shifter width biasing combined with the besttaper-angle wedge chrome-face wall.

The embodiment includes data biasing a determined desired circuitpattern. The data biasing might be done empirically, by trial and error,by using commercially available software such as Tempest available fromPanoramic Technology of Berkeley, Calif., using other software, or bysome combination of these. All of this by itself could constitute priorart methods, or yet to be developed methods for data biasing. In oneimplementation, an initial data bias circuit pattern to be used informing the ultimate desired circuit pattern on some substrate may, atleast initially,

IV. Example Embodiments of Other Mask Types Using the Slope SidewallOpaque Layer

The sloped sidewall be used in any other type masks. For example, inaddition to the alt-PSM single and double trench (no undercut) mask, theembodiment slope sidewall chrome pattern can be used on the masks shownin FIGS. 7A to 7D.

FIG. 7A shows SSCAAM: Sloped Sidewall Chrome Alternating Aperture Maskusing the embodiment sloped sidewall opaque layer.

FIG. 7B shows a Dual-trench Sloped Sidewall Chrome using the embodimentsloped sidewall opaque layer.

FIG. 7C shows Single-trench with undercut using the embodiment slopedsidewall light reducing layer.

FIG. 7D shows a double trench Sloped Sidewall Chrome with undercut usingthe embodiment.

It is possible to implement the slope sidewall opaque layer on othertype masks and the embodiment is not limited to the mask describedabove.

A. Additional Example Embodiments

In an example embodiment, a mask with an half tone layer with slopedsidewalls can be formed. That is for all the above embodiments, thechrome layer can be replaced by a half tone layer. The monitoringequations for the for the impedance wedge can be found in Graeme L.James, “Geometrical theory of diffraction for electromagnetic waves”,IEE Electromagnetic Waves Series, Institution of Electrical Engineers P.Peregr/Nov. 1, 1986 ISBN: 0863410626.

The example embodiment's mask could be a half tone PSM mask where thehalf tone layer has sloped sidewalls.

Another example embodiment mask can be a binary mask where some of thechrome edges (that define the openings) have sloped sidewalls.

Another example embodiment mask can be a rim shift PSM with slopedsidewalls.

Another example embodiment mask can be a outrigger PSM with slopedsidewalls.

B. Other Possible Shaped of the Slated Chrome Sidewall

The sloped (chrome) sidewall can have a rounded shaped (or non-planar ornon-linear shape. For example, the sloped chrome sidewall can have aslight non-planar shape due to mask fabrication process. We can takethese shapes of the slated chrome sidewall into consideration.

FIG. 8A shows an example of a rounded or curved shaped sidewall.

FIG. 8B shows another example of a convex or curved sloped chromesidewall. The sloped chrome sidewall can have a slight non-planar shapeddue to mask fabrication process.

A curved chrome sidewall can achieve a smaller sidewall angle 31 easier(in terms of mask fabrication) than a straight slanted chrome sidewall.

C. Non-Limiting Example Benefits

Some of the example embodiment can have some of the advantages:

-   -   PSM image imbalance correction can be implemented through the        application of shifter width biasing combined with the best        taper-angle wedge chrome-face wall.    -   Even a moderated amount of taper-angle helps to partially reduce        the image imbalance.    -   Through-focus imbalance magnitude is insensitive to shifter        biasing, but an optimal shifter bias makes it symmetric around        best focus.    -   Therefore, shifter bias of 90 nm coupled with angled-wedge        chrome-face wall can solve image imbalance without excessive        undercuts.

D. Non-Limiting Example Embodiments

Given the variety of embodiments of the present invention justdescribed, the above description and illustrations show not be taken aslimiting the scope of the present invention defined by the claims.

If advantageously needed in embodiments described above, the inventionwill be divided into a plurality of sections and embodiments and beexplained. And, except for particularly specifying cases, they havenothing to do with one another but one of them has something to do withmodifications, detailed explanation, supplementary explanation, or thelike relative to one portion or entire of the other.

Further, in the embodiments described above, except for a case ofcitation of number of elements or the like (containing number ofelements, a numerical value, a quantity, a numerical range or the like),a case of particular specification, a case of limit to specific numberspositively and in principle, and the like, the present invention is notlimited to the specific number and may be more than or less than orequal to the specific number.

Further, in the embodiments described above, it goes without saying thatcomponents (including processing steps or the like) thereof is notalways indispensable to the present invention except for a case ofparticular specification and a case of what is thought of asindispensable positively and in principle, and the like.

Similarly, in the embodiment described above, cases of citation ofshapes of components or the like, or/and positional relationships or thelike include something that is in fact close or similar to the shapes orthe like except for a case particular specification, and a case of whatis not thought of as indispensable positively and in principle, and thelike. This is also the same about the above-mentioned numeral values andranges.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention. It isintended to cover various modifications and similar arrangements andprocedures, and the scope of the appended claims therefore should beaccorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements and procedures.

1. A method for forming a semiconductor device comprising: providing asubstrate having a photoresist thereon; transmitting a light sourcethrough a mask having a pattern onto the photoresist, wherein the maskcomprises a mask substrate having first, second and third regions, thethird region is disposed between the first and second regions, and alight reducing layer over the mask substrate having a first opening overthe first region and a second opening over the second region, the firstand second openings having layer sidewalls, wherein the sidewalls of thelight reducing layer are slanted at an angle less than 90 degrees withthe plane of a top surface of the mask substrate; and developing thephotoresist to transfer the pattern of the mask to the photoresist. 2.The method of claim 1 wherein the light reducing layer comprises anopaque layer.
 3. The method of claim 1 wherein the light reducing layercomprises a half tone layer.
 4. The method of claim 1 wherein thesidewall angle is between 30 to 60 degrees.
 5. The method of claim 1further comprises forming a first trench in the first region of thesubstrate.
 6. The method of claim 5 wherein the first trench extendsunder the light reducing layer.
 7. The method of claim 5 furthercomprises forming a second trench in the second region of the substrate.8. The method of claim 7 wherein the first trench and the second trenchextend under the light reducing layer.
 9. The method of claim 1 furthercomprises forming the first and second openings with open dimensionsthat are biased smaller or larger from initial design sizes.
 10. Themethod of claim 1 wherein the sidewalls improve intensity balance of animage formed by light transmitted through the mask.
 11. A method offorming a device comprising: providing a first substrate with a layerthereon; transmitting a light source through a mask having a patternonto the layer, wherein the mask comprises a mask substrate havingfirst, second and third regions, the third region being disposed betweenthe first and second regions, a light reducing layer on a first majorsurface of the mask substrate, wherein the light reducing layer havingsidewalls defining openings to expose the first and second regions,wherein the sidewalls of the light reducing layer are slanted at anangle less than 90 degrees from a plane of the first major surface ofthe mask substrate; and developing the layer to transfer the pattern ofthe mask to the layer.
 12. The method of claim 11 wherein the lightreducing layer comprises an opaque layer.
 13. The method of claim 11wherein the light reducing layer comprises a half tone layer.
 14. Themethod of claim 11 wherein the sidewall angle is between 30 to 60degrees.
 15. The method of claim 11 further comprises forming a firsttrench in the first region of the substrate.
 16. The method of claim 15wherein the first trench extends under the light reducing layer.
 17. Themethod of claim 15 further comprises forming a second trench in thesecond region of the substrate.
 18. The method of claim 17 wherein thefirst trench and the second trench extend under the light reducinglayer.
 19. The method of claim 11 wherein the sidewalls improveintensity balance of an image formed by light transmitted through themask.
 20. A method of forming a device comprises: providing a firstsubstrate with a layer thereon; providing a mask comprising a secondsubstrate, wherein the second substrate comprises a transparent materialhaving first, second and third regions, the third region being disposedbetween the first and second regions; forming a light reducing layer ona first major surface of the second substrate; patterning the lightreducing layer to form a patterned light reducing layer having sidewallsdefining openings to expose the first and second regions; processing thepatterned light reducing layer to transform the sidewalls of thepatterned light reducing layer to angled sidewalls having an angle ofless than 90° from a plane of the first major surface of the secondsubstrate; and developing the layer to transfer the pattern of the maskto the layer.